Detective Superintendent Steve Fulcher was appointed lead investigator. His team found CCTV of Sian leaving the nightclub and walking in the direction of her home ten minutes’ away.

On the security footage they also saw a car repeatedly circling the area, stopping by Sian at 2.57am. The glare of the headlights in the night meant it was not possible to see why the car had stopped, but when it pulled away Sian had vanished.

Police believed she was in that car, but with the number plate obscured by headlamps in the dark, they didn’t know what car it was or who was driving it.

CCTV footage of Halliwell's taxi in Swindon. Credit: Wiltshire Police

And further analysis of Sian’s mobile phone movements painted a bleak picture. It had registered on the telephone mast in Baydon at 3.06am and then at Cadley near Marlborough in Savernake Forest at 3.25am. But Sian had no reason to go anywhere near either of these places at this time. And she wasn’t answering her phone, which was unlike her.

Helicopters with thermal-imaging equipment were sent up, search teams were sent into the forest. But with such a vast and rural area, the chances of finding Sian were slim.

Meanwhile, the CCTV footage was still being studied. An image analyst said he believed the suspect car was a ‘dark-toned’ Toyota Avensis estate made between 2003 and 2008. He also noted a slightly lighter area on the doors - perhaps a sticker or an advert?

Then the real breakthrough. Police noticed on the CCTV footage that one of their police cars was driving in the opposite direction to the suspect’s car - and it had an Automatic Number Plate Recognition (ANPR) camera.

It was just a fluke that the car was there, but analysis of the recordings showed a dark-coloured car with a minicab advert on the door for ‘Five Star’ and it had the registration number: AV07 FZF. A further check revealed this car to be owned by minicab driver Chris Halliwell.

Halliwell was put under 24-hour surveillance. DS Fulcher believed that watching the suspect was the best way of being led to where Sian was, whether she was alive or dead.

Surveillance teams watched Halliwell cleaning the rear passenger seat of his car with chemicals, they saw him throw away car seat covers in a wheelie bin across town and they saw him burning material in the countryside in the middle of the night.

Detectives visited Halliwell at his home. They took a DNA swab from his cheek and questioned him about his movements early on March the 19th. His answers were clearly inconsistent with the CCTV footage of his car in Old Town. The questioning officers said Halliwell appeared ‘stressed, close to tears and shaking.’

And DS Fulcher was using the media too to trap Halliwell. He issued a press release saying police believed they were ‘getting close,’ that dogs would be employed and that with the loss of daylight, the search would continue in the morning.

This tactic was designed to get Halliwell to respond after dark, to return to where he had abandoned Sian and move her body.

But the tactic failed. Halliwell stayed at home that evening. And DS Fulcher was getting worried.

The detective feared Halliwell, in an agitated and stressed state, may attempt suicide. If that happened, police would be unlikely to ever find Sian O’Callaghan.

That evening DS Fulcher did not sleep, desperately concerned that Halliwell would take his own life.

The next day surveillance teams reported to DS Fulcher that Halliwell was up and about, driving his minicab. But it appeared the detective’s fears had been realised. The undercover officers had seen the suspect buy an ‘overdose quantity’ of paracetamol.

That was when they decided to make the move. They didn’t know where Sian was, they didn’t know if she was alive or dead. They didn’t know for sure Halliwell was responsible. But they decided to arrest him then.

And his confession an hour later would reveal for certain Sian’s dreadful fate and that Halliwell was indeed the murderer.

Sian O'Callaghan. Credit: Facebook

Our reporter Robert Murphy has been following the case and looks back at the police investigation:

And Jonty Messer reports on how the community came together to help search for Sian: